1. Field of Invention
The present invention relates generally to piezoelectric ceramic transformers for power transfer circuits. More specifically, the present invention relates to the combination of piezoelectric transformers in a modular way to be use in a power supply system that delivers a plurality of selectable outputs at high voltage and high power. Different circuit topologies using piezoelectric transformers are provided that supply different voltages and powers rating to one or different loads requiring multiple voltages and power levels.
2. Description of the Prior Art
A common requirement to all phases of electronics that is the need for a supply of regulated DC power. Power supplies have been used in the state of the art to provide voltage and power to electric loads. Most power supplies are designed to meet some or all of the following requirements: (a) regulated output, (b) isolation, and (c) multiple outputs. A regulated output is required to maintain the output voltage within a specified tolerance for changes within a specified range in the input voltage and the output loading. Isolation is required to separate electrically the output from the input. Multiple outputs that may differ in their voltage and current ratings (power) and are isolated from each other are required to supply different voltages to different loads, or to the same load requiring multiple voltages.
In addition to these requirements, common goals on power supplies design are to reduce power supply size and weight and improve their efficiency. Furthermore, reduction of the electromagnetic interference (EMI) signature is a goal in systems where the integration of the power supply with other electronic components is required.
Traditionally, linear power supplies have been used. However, advances in the semiconductor technology have lead to switching power supplies, which are smaller and much more efficient compared to linear power supplies.
Referring to FIG. 1: A typical example of a linear power supply is shown in FIG. 1. Linear power supplies suffer of two major drawbacks. First, a low-frequency (50 Hz or 60 Hz) transformer is required to provide isolation between the input and the output and to deliver the output in the desired voltage range. Such transformers are larger in size and weight compared to high-frequency transformers. This represents a big penalty in the final size and weight of the power supply. Second, a linear transistor is connected in series that operate in its active region to regulate the output DC voltage. The linear transistor acts as an adjustable resistor where the voltage difference between the input and the desired output voltage appears across the transistor and causes power losses in it. Since the transistor operates in its active region, a significant amount of power is lost in it. Therefore, the overall efficiencies of linear power supplies are usually in a range of 30-60%.
Referring to FIG. 2: As opposed to linear power supplies, in switching power supplies, the transformation of DC voltage from one level to another is accomplished by using DC-to-DC converter circuits (or those derived from them). These circuits employ solid-state devices (transistors, MOSFETs, etc.), which operate as a switch: either completely off or completely on. Since the power devices are not required to operate in their active region, this mode of operation results in lower power dissipation. Increased switching speeds, higher voltage and current ratings, and a relatively lower cost of these devices are the factors that have contributed to the emergence of switching power supplies. Typical schematic of a switch-mode DC power supply is illustrated in FIG. 2.
Two major advantages of switching power supplies over linear power supplies are: (a) The switching elements (power transistors or MOSFETs) operate as a switch: either completely off or completely on. By avoiding their operation in their active region, a significant reduction in power losses is achieved. This results in higher energy efficiency in a 70-90% range. Moreover, a transistor operating in on/off mode has a much larger power-handling capability compared to its linear mode, so the size is reduced. (b) Since a high-frequency isolation transformer is used (as compared to a 50- or 60 Hz transformer in a linear power supply), the size and weight of switching supplies can be significantly reduced.
On the negative side, switching power supplies suffer of the drawback of using magnetic transformers at very high frequencies. This results in much higher EMI signature compare to linear power supplies. The magnetic transformer is still the largest component of the power supplies. This component limited the maximum power density and size of the power supplies.
Classically, wire wound-type electromagnetic transformers have been used for generating a transformation from one input voltage level to a different voltage level. Step-up and step-down applications can be found in the prior state of the art. For instance, wire wound-type electromagnetic transformers have been used for step-up applications for generating high voltage in internal power circuits of devices such as televisions, fluorescent lamp ballasts, CCFL backlighting, and others. Also, electromagnetic transformers have been using in step-down applications such as battery chargers for lap-top computers, cell phones, and other similar applications.
Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce. In addition to being large in size and weight, wound transformers create EMI which can disrupt the performance of other circuits and components in proximity to the transformer, which is a major issue in compact portable devices having a multitude of circuitry in a small packing area, such as laptop computers, PDA's, camcorders, and other handheld devices. Furthermore, in view of high frequency applications and compact size application, the electromagnetic transformer has many disadvantages related to the materials used in their manufacturing. Magnetic materials used for the cores of transformers have two types of electrical losses, eddy current loss due to finite electrical conductivity and hysteresis (magnetic) loss. A third type of loss is related to the windings of the transformer. These windings are made from copper wire, which copper losses include not only DC resistance loss, and additional ohmic loss caused by non-uniform current density concentrations arising from the proximity effect and skin effect. These losses, specifically hysteresis and skin effect losses increase in high frequency applications and force the designing engineer to over-design the magnetic components which, in turn, affects the final size. Furthermore, wire-wound transformers also require winding isolation material, which also affects the final size of the component. This is even a bigger issue in high voltage transformers where dielectric breakdown risk between high voltage and low voltage wiring limits the minimum thickness of the isolation material used. Furthermore, the maximum permissible temperature of a transformer is approximately 100° C. and is limited by both magnetic material and winding isolation material considerations. This temperature limit along with the surface-to-ambient thermal resistance of the component limit the average power dissipation density (W/cm3) in the component. This power dissipation density limit translates into a maximum current density limit in the copper winding and a maximum peak AC flux density in the core material, and thus in the maximum power density that the wire-wound transformer can supply.
To remedy this and many other problems of wire-wound transformers, piezoelectric ceramic transformers (or PTs) utilizing the piezoelectric effect have been provided in the prior art. In contrast to electromagnetic transformers, PTs have a sharp frequency characteristic of the output voltage to input voltage ratio, which has a peak at the resonant frequency. This resonant frequency depends on the material constants and dimensions and shape of materials of construction of the transformer including the piezoelectric ceramics and electrodes. Furthermore, PTs have a number of advantages over general electromagnetic transformers. The size of PTs can be made much smaller than electromagnetic transformers of comparable transformation ratio, PTs are nonflammable, and produce no electromagnetically induced noise.
Piezoelectric transformer technology has evolved around three fundamentally different PT families: “Rosen-type” PTs, “Thickness-type” PTs, and “Laminated-type” PTs. Rosen-type PTs were the first PTs developed and are characterized by a common area for the input and the output section corresponding to the transversal area of the ceramic body. This area is typically transverse to the propagation direction of the acoustic standing front-wave. Furthermore, the input to output coupling area is also, typically, the nodal area of the PT, i.e., the area with no deformation and higher stress levels. Rosen-PTs have been proposed in rectangular, circular, or annular shapes. Thickness-type PTs make use of discs or plates vibrating in the thickness mode. In these PTs, the coupling areas between the input and the output are the major surfaces of the input and output sections. In these PTs, the nodal point is established in the coupling area. In laminated-type PTs the input and output are also acoustically coupled at their major surfaces. However, in these types of PTs the nodal point does not separate the input from the output section. The coupling area between input and output is NOT a nodal area of the PT, but it moves with the vibration of the PT.
The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type PT comprises a flat ceramic slab 20 appreciably longer than it is wide and substantially wider than it is thick. Rosen-type PTs have been proposed in various forms and configurations, including rings, flat slabs and the like as disclosed in U.S. Pat. No. 2,830,274 (1958) by C. Rosen el al., U.S. Pat. Nos. 3,562,792 (1971) and 3,764,848 (1973) both by D. Berlincourt, U.S. Pat. No. 4,767,967 (1988) by Tanaka et al, U.S. Pat. No. 5,736,807 by Hakamata et al., and others. Typical examples of a prior Rosen-type PTs are illustrated in FIGS. 3 and 4. In the case of FIG. 3, the piezoelectric body 20 is in the form of a flat slab that is considerably wider than it is thick, and having greater length than width.
As shown in FIG. 3, a piezoelectric body 20 is employed having some portions polarized differently from others. A substantial portion of the slab 20, the generator portion 22 to the right of the center of the slab is polarized longitudinally, and has a high impedance in the direction of polarization. The remainder of the slab, the vibrator portion 21 is polarized transversely to the plane of the face of the slab (in the thickness direction) and has a low impedance in the direction of polarization. In this case the vibrator portion 21 of the slab is actually divided into two portions. The first portion 24 of the vibrator portion 21 is polarized transversely in one direction, and the second portion 26 of the vibrator portion 21 is also polarized transversely but in the direction opposite to that of the polarization in the first portion 24 of the vibrator portion 21.
In order that electrical voltages may be related to mechanical stress in the slab 20, electrodes are provided. If desired, there may be a common electrode 28, shown as grounded. For the primary connection and for relating voltages at opposite faces of the low impedance vibrator portion 21 of the slab 20, there is an electrode 30 opposite the common electrode 28. For relating voltages to stresses generated in the longitudinal direction in the high impedance generator portion 22 of the slab 20, there is a secondary or high-voltage electrode 35 on the end of the slab for cooperating with the common electrode 28. The electrode 35 is shown as connected to a terminal 34 of an output load 36 grounded at its opposite end.
In the arrangement illustrated in FIG. 3, a voltage applied between the electrodes 28 and 30 of the low impedance vibrator portion 21 is stepped up to a higher voltage between the electrodes 28 and 35 in the high impedance generator portion for supplying the load 36 at a much higher voltage than that applied between the electrodes 28 and 30. The applied voltage causes a deformation of the slab through proportionate changes in the x-y and y-z surface areas. More specifically, the Rosen PT is operated by applying alternating voltage to the drive electrodes 28 and 30, respectively. A longitudinal vibration is thereby excited in the low impedance vibrator portion 21 in the transverse effect mode (d31 mode). The transverse effect mode vibration in the low impedance vibrator portion 21 in turn excites a vibration in the high impedance generator portion 22 in a longitudinal effect longitudinal vibration mode (g33 mode). As the result, high voltage output is obtained between electrode 28 and 35. Typically, Rosen-type PTs are designed to operate under half wavelength (lambda/2) or three half wavelength (3×lambda/2). The total length of the PT of FIG. 3 determines the value of the operational resonance frequency of the PT.
An inherent problem of such prior Rosen-PTs is that they have relatively low power transmission capacity. This disadvantage of prior PTs relates to the fact that little or no mechanical advantage is realized between the vibrator portion 21 of the device and the driver portion 22 of the device. Because the driver and vibrator portions each is intrinsically a part of the same electroactive member, the transmission of energy between portions is limited to the transverse area of the longitudinal body. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices.
A second problem of Rosen-PTs, such the one of FIG. 3, is the non-symmetric structure in the length direction. Since the polarization direction in ceramic piezoelectric materials relies significantly on the material properties, such as in the stiffness, dielectric permitivity, and piezoelectric properties, the mechanical behavior of the Rosen-PT will not be mechanically symmetrical in the length direction. As a result, Rosen-PTs show spurious bending resonances around the main resonance frequency, specifically when thin bodies are used. This bending resonance may interfere with the main resonance of the PT and thus diminish the efficiency of the PTs. Additionally, the spurious bending resonance may affect the tracking circuitry of the Rosen-PT and may render the PT useless in practice.
Additionally, since the transmitted power density is limited by the strain endurance of the piezoelectric material, Rosen-type PTs are limited in power to the maximum permissible tensile stresses in the nodal transversal area, which is typically very small. As consequence of this, Rosen-PTs become mechanically weak and may suffer fracture in the nodal transversal area.
Another problem with prior Rosen PTs is that the input and output capacitances depend upon the total dimension of the ceramic bar used. Once the dimensions of the slab are selected, the value of the output capacitance design is fixed since it depends on the thickness of the bar and the half of the total length of the bar for Rosen-type PT operating in the lambda-half mode.
Another drawback of conventional Rosen-type PTs is that since the electrode of the high voltage section is located in the loop of vibration, i.e., in the vibrating direction, connection of the external terminals adversely affects vibration or largely degrades reliability.
Referring to FIG. 5: The second family of PTs is the “Thickness-type PT”, which are PTs polarized and vibrating in the thickness direction (i.e., vibrations are parallel to the direction of polarization of the layers). Illustrative of such thickness mode vibration PTs is the device of U.S. Pat. No. 5,118,982 to Inoue shown in FIG. 5. A thickness mode vibration PT typically comprises a low impedance portion 11 and a high impedance portion 12 stacked on each other. The low impedance portion 11 and the high impedance portion 12 of the thickness mode PT typically comprises a series of laminate layers of ceramic alternating with electrode layers. Each portion is composed of at least two electrode layers and at least one piezoelectric material layer. Each of the piezoelectric ceramic layers of the low impedance portion 11 and the ceramic layer of the high impedance portion 12 are polarized in the thickness direction (perpendicular to the plane of the interface between the ceramic layers). Every alternate electrode layer in each portion 11 or 12 may be connected to each other and to selected external terminals.
The thickness mode PT of FIG. 5 comprises a low impedance vibrator portion 11 including a plurality of piezoelectric layers 211 through 214 and a high impedance vibrator portion 12 including a piezoelectric layer 222, each of the layers being integrally laminated, and caused to vibrate in thickness-extensional mode. The low impedance portion 11 has a laminated structure which comprises multi-layered piezoelectric layers 211 through 214 each being interposed between electrodes including the top surface electrode layer 201 and internal electrode layers 231 through 234. The high impedance portion 12 is constructed of the bottom electrode layer 202, an internal electrode layer 234 and a single piezoelectric layer 122 interposed between both electrode layers 202 and 234. Polarization in each piezoelectric layer is, as indicated by arrows, in the direction of thickness, respectively. In the low impedance portion 11, alternating piezoelectric layers are polarized in opposite directions to each other. The polarization in the high impedance portion 12 is also in the direction of thickness. The PT has a common electrode 234 to which one terminal 16 of each portion is connected. The total thickness of the PT of FIG. 5 is restricted to a half wavelength (lambda/2) or one full wavelength (lambda) of the drive frequency.
When an alternating voltage is applied to the electrode layers across the ceramic layer of the vibrator portion 11, a vibration is excited in the ceramic parallel to the direction of the polarization of the layers in the longitudinal vibration mode (d33 mode). This vibration of the low impedance portion 11 excites a vibration (g33 mode) in the high impedance portion 12. As the high impedance portion 12 vibrates, the g33 mode deformation of the high impedance portion 12 generates an electrical voltage across the electrodes of the high impedance portion 12. When operating the PT in the thickness-extensional mode with a resonance of lambda/2 mode (both end free fundamental mode) or lambda mode (both end-free secondary mode), the PT may operate in a frequency range of 1-10 MHz.
An inherent problem with prior thickness mode PTs is that the thickness mode resonant frequency is too high for some applications. Although the high frequency operation initially promotes higher power efficiency, the power loss generated by circulating current in the PT decreases significantly the PT efficiency and consequently increases the heat generation, limiting the maximum power available.
Another problem with prior thickness mode PTs is the losses affecting the driving switching inverter used to drive them, which limit the application of these PTs to high power applications.
Another problem with prior thickness mode PTs is their limitation to reach high output voltages, due to their thin thickness and low output impedance, which leaves them out of the scope of the present invention.
Referring now to FIGS. 6 and 7: The third family of PTs is the “Laminate-type PTs”. Two types of laminated-type PTs include radial-type laminated piezoelectric transformers and longitudinal-type piezoelectric transformers. In general, a laminated PT typically comprises a high impedance portion 60 and a low impedance portion 40 stacked on each other. The vibrator portion 21 is polarized transversely to the plane of the face of the slab (in the thickness direction) and has a high impedance in the direction of polarization. The high impedance portion 60 is polarized in the thickness direction. The low impedance portion 40 is divided in two parts by a belt electrode printed in the center of the slab. Each portion 41 and 42 is polarized in the longitudinal direction. Laminated PTs have been demonstrated to overcome the power limitations of Rosen type PTs and thickness-type PTs. Radial laminated PTs are used for step-down applications, such as battery chargers for laptops and cell phones, as well as in ballast for fluorescents. Longitudinal laminated PTs are preferred in step-up transformers.
In step-down laminated-type PTs, such as in FIG. 6, the input portion (driver section) has higher impedance (lower capacitance) than the output portion (generator section). Thus, the output voltage of the transformer has a lower value that the input voltage applied to the driver section. Step-down laminated-type PTs are typically made using radial type PTs. Illustrative of such step-down laminated PTs is the device disclosed in U.S. Pat. No. 5,834,882 to Richard P. Bishop (1998), U.S. Pat. No. 6,333,589 (2001) to Inoi, and U.S. Application Patent 20030067252 by Alfredo Vazquez Carazo, shown in FIG. 6
In step-up laminated-type PTs, such as in FIG. 7, the input portion (driver section) has a lower impedance (higher capacitance) than the output portion (lower capacitance). Thus, the output voltage of the PT has a higher value that the input voltage applied to the driver section. Step-up laminated-type PTs are typically made using longitudinal type PTs. Illustrative of such step-up laminated PTs is the device disclosed in U.S. Pat. No. 6,326,718 (2001) by Boyd et al. (The Face Companies), shown in FIG. 7.
A global problem of the prior state of the art of the piezoelectric transformer technology is the impossibility of providing multiple outputs that may differ in their voltage and current rating. In this prior state of the art, the piezoelectric transformer has a limitation of providing a single output voltage to drive a specific load.
Thus, an inherent problem of the prior state of the art of power supplies using piezoelectric transformer technology is the limitation to achieve very high voltages beyond the manufacturing possibilities of a simple sample of PT.
Another problem of the prior state of the art of power supplies using piezoelectric transformer technology is the limitation to achieve very high power levels beyond the manufacturing possibilities of a simple sample of PT.
Another problem of the prior state of the art of power supplies using piezoelectric transformer technology is the impossibility to provide at the same time high power and high voltage levels at the same time to the load.
Another limitation of the prior state of the art of power supplies using piezoelectric transformer technology is the impossibility of providing multiple outputs to drive different loads.
Another limitation of the prior state of the art of power supplies using piezoelectric transformer technology is the impossibility of providing multiple outputs which differ in their voltage and current rating.
Another limitation of the prior state of the art of power supplies using piezoelectric transformers technology is the impossibility of providing multiple outputs which are isolated from the input.
Accordingly, it would be desirable to provide designs for combining piezoelectric transformers that has a higher step-up ratio capacity beyond the capacity of single units of piezoelectric transformers.
It would also be desirable to provide designs for combining piezoelectric transformers that can provide at the same time high levels of voltage and high level of power beyond the capacity of single units of piezoelectric transformers
It would also be desirable to provide designs for combining piezoelectric transformers that has a higher power transmission capacity than similarly sized prior power supplies.
It would also be desirable to provide designs for combining piezoelectric transformers that capable of providing a plurality of outputs which may differ in their voltage and current ratings and may be isolated from each other and from the input.
It would also be desirable to provide designs for combining piezoelectric transformers has a lower signature of EMI compared to prior state of the art power supplies.
It would also be desirable to provide a circuit of the character described that is highly compact and efficient.